There are known optical discs called as DVD or BD (Blu-ray disc), as examples of the commercially available high-density and large-capacity optical information recording media. In recent years, the optical discs have become widely used as recording media for recording images, music, and computer-readable data. There also has been proposed an optical disc having plural recording layers, as disclosed in Patent literature 1 and Patent literature 2, to further increase the recording capacity.
FIG. 13 is a diagram showing an arrangement of a conventional optical recording medium and optical head device. An optical recording medium 401 includes a first information recording surface 401a closest to a surface 401z of the optical recording medium 401, a second information recording surface 401b second closest to the surface 401z of the optical recording medium 401, a third information recording surface 401c third closest to the surface 401z of the optical recording medium 401, and a fourth information recording surface 401d farthest from the surface 401z of the optical recording medium 401.
A divergent beam 70 emitted from a light source 1 is transmitted through a collimator lens 53, and incident into a polarized beam splitter 52. The beam 70 incident into the polarized beam splitter 52 is transmitted through the polarized beam splitter 52, and converted into circularly polarized light while being transmitted through a quarter wavelength plate 54. Thereafter, the beam 70 is converted into a convergent beam through an objective lens 56, transmitted through a transparent substrate of the optical recording medium 401, and collected on one of the first information recording surface 401a, the second information recording surface 401b, the third information recording surface 401c, and the fourth information recording surface 401d formed in the interior of the optical recording medium 401.
The objective lens 56 is so designed as to make a spherical aberration zero at an intermediate depth position between the first information recording surface 401a and the fourth information recording surface 401d. A spherical aberration corrector 93 shifts the position of the collimator lens 53 in an optical axis direction. Thereby, spherical aberration resulting from collecting light on the first through the fourth information recording surfaces 401a through 401d is removed.
An aperture 55 restricts the opening of the objective lens 56, and sets the numerical aperture NA of the objective lens 56 to 0.85. The beam 70 reflected on the fourth information recording surface 401d is transmitted through the objective lens 56 and the quarter wavelength plate 54, converted into linearly polarized light along an optical path displaced by 90 degrees with respect to the outward path, and then reflected on the polarized beam splitter 52. The beam 70 reflected on the polarized beam splitter 52 is converted into convergent light while being transmitted through a light collecting lens 59, and incident into a photodetector 320 through a cylindrical lens 57. Astigmatism is imparted to the beam 70 while the beam 70 is transmitted through the cylindrical lens 57.
The photodetector 320 has unillustrated four light receiving sections. Each of the light receiving sections outputs a current signal depending on a received light amount. A focus error (hereinafter, called as FE) signal by an astigmatism method, a tracking error (hereinafter, called as TE) signal by a push-pull method, and an information (hereinafter called as RF) signal recorded in the optical recording medium 401 are generated, based on the current signals. The FE signal and the TE signal are amplified to an intended level, subjected to phase compensation, and then supplied to actuators 91 and 92, whereby focus control and tracking control are performed.
In this example, the following problem occurs, in the case where the thickness t1 between the surface 401z of the optical recording medium 401 and the first information recording surface 401a, the thickness t2 between the first information recording surface 401a and the second information recording surface 401b, the thickness t3 between the second information recording surface 401b and the third information recording surface 401c, and the thickness t4 between the third information recording surface 401c and the fourth information recording surface 401d are equal to each other.
For instance, in the case where the beam 70 is collected on the fourth information recording surface 401d to record or reproduce information on or from the fourth information recording surface 401d, a part of the beam 70 is reflected on the third information recording surface 401c. The distance from the third information recording surface 401c to the fourth information recording surface 401d, and the distance from the third information recording surface 401c to the second information recording surface 401b are equal to each other. Accordingly, the part of the beam 70 reflected on the third information recording surface 401c forms an image on a backside of the second information recording surface 401b, and reflected light from the backside of the second information surface 401b is reflected on the third information recording surface 401c. As a result, the light reflected on the third information recording surface 401c, the backside of the second information recording surface 401b, and the third information recording surface 401c may be mixed with reflected light from the fourth information recording surface 401d to be read.
Further, the distance from the second information recording surface 401b to the fourth information recording surface 401d, and the distance from the second information recording surface 401b to the surface 401z of the optical recording medium 401 are equal to each other. Accordingly, a part of the beam 70 reflected on the second information recording surface 401b forms an image on the backside of the surface 401z of the optical recording medium 401, and reflected light from the backside of the surface 401z is reflected on the second information recording surface 401b. As a result, the light reflected on the second information recording surface 401b, the backside of the surface 401z, and the second information recording surface 401b may be mixed with reflected light from the fourth information recording surface 401d to be read.
As described above, there is a problem that reflected light from the fourth information recording surface 401d to be read is superimposed and mixed with reflected light which forms an image on the backside of the other surface, with the result that information recording/reproducing is obstructed. Light containing reflected light which forms an image on the backside of the other surface has a high coherence, and forms a brightness/darkness distribution on a light receiving element by coherence. Since the brightness/darkness distribution is varied depending on a change in phase difference with respect to reflected light from the other surface, resulting from a small thickness variation of an intermediate layer in an in-plane direction of an optical disc, the quality of a servo signal and a reproduction signal may be considerably deteriorated. Hereinafter, the above problem is called as a back focus problem in the specification.
In order to prevent the back focus problem, Patent literature 1 discloses a method, wherein the interlayer distance between the information recording surfaces is gradually increased in the order from the surface 401z of the optical recording medium 401 so that a part of the beam 70 may not form an image on the backside of the second information recording surface 401b and the backside of the surface 401z simultaneously when the beam 70 is collected on the fourth information recording surface 401d to be read. The thicknesses t1 through t4 each has a production variation of ±10 μm. It is necessary to set the thicknesses t1 through t4 to different values from each other, also in a case where the thicknesses t1 through t4 are varied. In view of this, a difference in the thicknesses t1 through t4 is set to e.g. 20 μm. In this example, the thicknesses t1 through t4 are respectively set to 40 μm, 60 μm, 80 μm, and 100 μm, and the total interlayer thickness t(=t2+t3+t4) from the first information recording surface 401a to the fourth information recording layer 401d is set to 240 μm.
In the case where the thickness of a cover layer from the surface 401z to the first information recording surface 401a, and the thickness from the fourth information recording surface 401d to the first information recording surface 401a are equal to each other, light reflected on the fourth information recording surface 401d is focused on the surface 401z, and reflected on the surface 401z. The light reflected on the surface 401z is reflected on the fourth information recording surface 401d, and guided to the photodetector 320. A light flux which forms an image on the backside of the surface 401z does not have information relating to pits or marks, unlike a light flux which forms an image on the backside of the other information recording surface. However, in the case where the number of information recording surfaces is large, the light amount of light returning from the information recording surfaces is reduced, and the reflectance of the surface 401z is relatively increased. As a result, coherence between a light flux reflected on the backside of the surface 401z, and a light flux reflected on a targeted information recording surface to be recorded or reproduced is generated in the similar manner as in the case of a light flux reflected on the backside of the other information recording surfaces, which may considerably deteriorate the quality of a servo signal and a reproduction signal.
In view of the above problem, Patent literature 2 proposes a distance between information recording layers (information recording surfaces) of an optical disc. Patent literature 2 discloses the following structure.
An optical recording medium has four information recording surfaces, wherein the first through the fourth information recording surfaces are defined in the order from a side closest to a surface of the optical recording medium. The distance from the medium surface to the first information recording surface is set to 47 μm or less. The thicknesses of intermediate layers between the first through the fourth information recording surfaces are combination of a range from 11 to 15 μm, a range from 16 to 21 μm, and a range of 22 μm or more. The distance from the medium surface to the fourth information recording surface is set to 100 μm. The distance from the medium surface to the first information recording surface is set to 47 μm or less, and the distance from the medium surface to the fourth information recording surface is set to 100 μm.
An optical disc system is adapted to detect light incident from a medium surface and reflected on an information recording surface. Accordingly, a refractive index of a transparent material constituting a transparent member from the medium surface where light is transmitted to the information recording surface also affects the quality of a servo signal and a reproduction signal. However, there is no consideration and description about the refractive index in the disc structures disclosed in Patent literature 1 and Patent literature 2. Thus, both of the publications do not consider an influence of a refractive index of a transparent material to the quality of a servo signal and a reproduction signal.